Integrated circuits are typically fabricated in substrate form. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
The integrated circuits are typically fabricated using a basic process flow, that includes steps such as depositing a layer, forming a layer of photoresist over the layer, exposing and developing a pattern into the photoresist, and then modifying in some manner the underlying exposed layer through the photoresist, such as by etching the layer, after which the remaining photoresist layer is removed. Although this basic flow can be modified in a number of different ways, this basic process is repeatedly performed to create additional layers on the substrate, until the integrated circuits on the substrate are fabricated.
There is continual pressure to reduce the size of integrated circuits. This pressure comes from a number of different sources, such as the desire to have more circuitry within a given space, increase the number of dice on a substrate to as to increase profits, and reduce power consumption. As the critical dimensions of the integrated circuits continue to decrease, the problems that must be overcome in the fabrication process continue to increase.
For example, one problem is the proper alignment—from layer to layer—of the features that are formed in the various layers. At times past, such proper alignment could be ensured by merely producing a proper mask set, and paying strict attention to the alignment of the features from layer to layer. However, as new lithography processes are introduced that stretch the optical resolution of the light that is used to expose the photoresist, additional considerations need to be taken into account.
What is needed, therefore, is a system for overcoming problems such as those described above, at least in part.